The present invention relates to a hazardous waste incinerator, and more particularly to a multi-sectional rotary kiln incinerator used to treat variable waste streams having different caloric value, water content and slagability.
The use of rotary kilns for the incineration of hazardous waste is known. U.S. Pat. No. 4,734,166 to Angelo II discloses a rotatable cylindrical kiln. Rotary kilns are widely used due to their versatility to transport a wide variety of solid waste throughout the kiln chamber, making kilns suitable for treating the majority of solid waste streams with minimum preparation and shredding.
Many high BTU solid waste streams are decontaminated in conventional rotary kilns by burning with an additional oxidizing gas, such as air being delivered to the surface of a rotating bed. For such high BTU waste streams, the major part of heat needed to vaporize and thermally destroy hazardous components is generated by oxidation of the combustible components of the waste stream so that the release of the major portion of the heat is controlled by the distribution of the additional oxidizing gas along the kiln length. If the solid waste includes an appreciable amount of water, an initial drying is conducted by the use of drying heat being transferred to the bed from auxiliary burner(s). Auxiliary burners are also fired in the kiln to initially ignite the bed and to insure the continuous presence of a flame in the kiln to prevent the danger of flame loss and possible explosion.
The demand on the auxiliary burner for heat input significantly increases when high BTU waste has a high moisture content. Under such conditions, the role of auxiliary burners as a heating source becomes important to accomplish the rapid and effective drying of the waste layer prior to ignition. After drying and the ignition of high BTU waste, the heat being released by the waste burning is essential to support the burning and, therefore, to make the incineration process autogenous.
To provide for active waste burning, an adequate level of available oxygen and the rate of oxygen mass exchange should be maintained between a hot, dried waste residue layer and the kiln atmosphere. An additional oxidizing gas such as air, oxygen-enriched air, or pure oxygen has to be directed into the kiln interior to provide the necessary concentration of oxygen in the furnace atmosphere. Typically, the additional oxidizing gas is injected at the waste charging end of the kiln. Such injection, however, provides a negative impact on waste drying due to the substantial absorption of the heat being released by the auxiliary burner(s) by this additional oxidizing gas.
Therefore, the incineration process creates a significant variance in heat release and oxygen consumption along the rotary kiln length. Unfortunately, the conventional way of introducing both the auxiliary heat and the additional oxidizing gas responsible for heat release by the waste is limited to the rotary kiln ends. Such limitation reduces the controllability of heat input and oxygen mass transfer, which reduces the potential performance of rotary kiln incinerators.
When low BTU solid wastes, such as contaminated soil, are treated in a rotary kiln, a major part of the heat needed to vaporize and thermally destroy the hazardous constituents of the waste should be supplied by an auxiliary heat source. Therefore, high heat flux from one or more auxiliary burners must be maintained inside the rotary kiln to provide high levels of throughput, which are needed to make the incineration method of soil decontamination economically attractive.
To intensify heat transfer from the auxiliary flame envelope to the load located in the rotary kiln, the mixing process between fuel and oxidizing gas should be promoted by the burner designed to achieve a concentrated heat release inside of the flame envelope so as to maximize the time of contact between hot combustion products and the solid residue in the kiln. At the same time, due to the uneven heat flux from the flame envelope to the solid residue, an optimized distribution of heat flux is needed to maximize furnace throughput.
The ability of a solid layer of low BTU soil to conduct heat delivered to its surface by auxiliary burner(s) varies along the length of the kiln. The solid layer at the charging end of the rotary kiln may absorb a higher level of heat flux during the initial part of the heating cycle due to the presence of moisture and the higher temperature differential between the solid waste and the combustion products of the auxiliary burner, which is typically installed at the waste charging end. The ability of the solid layer to absorb heat is reduced as this soil increases in temperature travelling along the kiln. The kiln rotation speed is typically maintained at a relatively low level to minimize carry-over of solid waste particles. Due to the increasing the temperature of the soil bed surface been exposed to kiln atmosphere as it travels through the kiln, the heat flux from auxiliary burner(s) should be substantially reduced to prevent local overheating which causes slag formation and undesirable solid residue agglomeration. Because of this, the process of incineration of contaminated soil in a conventional rotary kiln involves a higher level of heat flux is typically delivered by the flame envelope(s) of auxiliary burners at the waste charging end and a substantially lower heat flux delivered by the flue gases of this burner(s) along the rest of the kiln length.
This especially impacts kilns used to decontaminate a material with low thermal conductivity such as soil. In such cases, the fear of local overheating forces the rotary kiln designers and operators to keep a reduced heat flux from the burners. This results in the loss of potential throughput capacity.
The flexibility of controlling the heat flux from combustion products of the auxiliary flame in rotary kilns is limited due to the restrictions on the placement of auxiliary burners. The location of an auxiliary burner is possible only at either end of the rotating chamber due to rotation of the kiln sidewall. Such burner locations make it difficult to optimize heat flux along the entire kiln length from the hot combustion products of auxiliary burner(s) to the solid bed having low thermal conductivity.
The process of incineration of high BTU wastes provides for the major heat release by complete or partial oxidation of the combustible components of a waste stream by using additional oxidizing gas. This oxidation process should also be controllable in order to maintain the desired heat input and temperature distribution along the entire kiln length. This should be accomplished by improved control over distribution of additional oxidizing gas inside of the kiln and preferably along the entire kiln length.
The introduction of additional oxidizing gas is also limited due to the rotation of the kiln sidewalls which restricts the placement of injectors for said oxidizing gas introduction. The introduction, at the waste charging end, of the entire volume of additional oxidizing gas needed for burning the solid residue along the entire length of the kiln provides as increase of actual gaseous volume and high superficial gas velocities. This results in an increase of waste particle carry-over from the rotary kiln.